[ 123 ]

exposed. If it is suspected to be gilt with a thick coatof metal finer than the internal part, it should be raisedwith a graver, to some depth, that the exteriour coat maybe broken through : cutting the piece in two is a lesscertain way of discovering this abuse; the outer coat beingfrequently drawn along by the sheers or chisel, so as tocover the divided parts.

The metallic compositions, made to resemble gold incolour, are readily known by means of a drop or two ofaqua fortis, which has no effect upon gold, but dissolvesor discharges the marks made by all its known imitations.That the touchstone may be able to support this trial, itbecomes a necesiary character of it not to be corrosible byacids; a character which strews it to be essentially differ-ent from the marbles, whereof it is by many writersreckoned a species. If gold is debased by an admixtureof any considerable quantity of these compositions, aquafortis will in this cafe also discharge so much of the markas was made by the base metal, and leave only that of thegold, which will now appear discontinued or in specks.Silver and copper are in like manner eaten out from goldon the touchstone, and hence some judgement may thusbe formed of the fineness of the metal from the proportionof the remaining gold to the vacuities.

Ercker observes that hard gold appears on the touch-stone less fine than it really is. It may be presumed thatthis difference does not proceed from the simple hard-ness : but from the hardness being occasioned by an ad-mixture os such metallic bodies, as debase the colour ina greater degree than an equal quantity of the commonalloy. Silver and copper are the only metals usually foundmixed with gold whether in bullion or in coins; and theonly ones, whose quantity is attempted to be judged of bythis method of trial.

R 2

The